Typically, decanter centrifuges use an electric motor (either DC or AC) to rotate the bowl of the centrifuge. The rotation of the bowl creates a centrifugal force for separation of the constituent parts of the feed slurry. A separate driving element is provided to rotate the conveyor portion of the decanter centrifuge at a differential rotational speed with respect to the rotation of the bowl. This differential speed of the conveyor creates a discharging force on the separated heavy phase or solids material and moves same toward the discharge outlets in the bowl.
In order to start the rotation of a decanter centrifuge and bring it up to operational speed, the inertia of the centrifuge must be overcome. In addition, the windage resistance and frictional resistance of the centrifuge during rotation (which changes with the speed of rotation) must be overcome. The inertial and other resistance may place a substantial burden on the motor drive due to the high current and long acceleration time required of the motor. This burden on the motor may exceed that resulting from the maintenance of the operational speed of the centrifuge.
Previously, decanter centrifuges were of a size which permitted the application of standard motor technology for start-up and for normal operation. If the centrifuge exceeded the limitations of a particular motor, during starting or normal operation, other elements such as fluid couplings, transmissions, variable frequency drives, direct current motors, mechanical or hydraulic clutches were provided. However, in some applications these elements are not desirable or preferred. Alternatively (or in addition to the above elements), special heavy duty motors were provided. However, as the size and speed of the centrifuge bowl has increased, the burden on the motor drive has substantially increased.
The "heavy duty" construction of the motor includes additional structure and special materials for purposes of withstanding the heat build-up created during start-up. The cost of these special "heavy duty" motors substantially increases with size due to the significant increase in the quality, strength and amount of the materials required to create the desired properties of the motor. Additionally, the size of the motor and the size of the associated support frame substantially increase.
The increase in "size" of a "heavy duty" motor may also result in a loss of efficiency at normal operational speeds. This may be due to the special design for thermal load at start-up. Preferably, a motor would allow start-up to occur and then operate at full efficiency at normal operating speeds.